Biooils are liquid products obtained by thermochemical liquefaction of lignocellulosic biomass materials. Thermochemical methods generally convert biomass into liquid, gaseous and solid products. Among them, so-called fast or flash pyrolysis methods aim to maximize the liquid yield. In fast pyrolysis, biomass, possibly finely divided, is heated rapidly to temperatures above about 400° C. and the liquid products condensed as biooil. Ringer et al. (Large-Scale Pyrolysis Oil Production: A Technology Assessment and Economic Analysis, M. Ringer, V. Putsche, and J. Scahill, NREL Technical Report NREL/TP-510-37779, November 2006) have discussed the various technologies that have been deployed for large scale biomass fast pyrolysis. They include bubbling fluidized beds, circulating fluidizing beds, ablative pyrolysis, vacuum pyrolysis, and rotating cone pyrolysis reactors. They point out that so long as the heat transfer requirements are met the chemical nature of the biooil product will be reasonably consistent between pyrolysis processes.
Prima facie, biooils could, in principle, provide low cost renewable liquid fuels; indeed their use as fuel for boilers, as well as for stationary gas turbines and diesels, have all been demonstrated. Furthermore fast pyrolysis has been demonstrated at fairly large scales, of the order of several hundred tons per day. Nevertheless there has not been any significant commercial uptake of this technology.
The reasons relate mostly to the poor physical and chemical properties of biooils in general and fast pyrolysis biooils in particular. For example, some of the undesirable properties of pyrolysis biooils are: (1) corrosivity on account of their high water and acidic contents; (2) a relatively low specific calorific value on account of the high oxygen content, which typically is around 40% by mass; (3) chemical instability on account of the abundance of reactive functional groups like the carbonyl group and phenolic groups that can lead to polymerization on storage and consequent phase separation; (4) a relatively high viscosity and susceptibility to phase separation under high shear conditions, for instance in a nozzle; (5) incompatibility with, on account of insolubility in, conventional hydrocarbon based fuels; (6) adventitious char particles, which will always be present in unfiltered biooil to a greater or lesser degree, can cause blockages in nozzles and pipes. All these aspects combine to render biooil handling, shipping storage and usage difficult and expensive and to make integration into current heat and power producing systems and technologies problematic.
The economic viability of biooil production for energy applications therefore depends on finding appropriate methods to upgrade it to a higher quality liquid fuel at a sufficiently low cost. Indeed, considerable effort has been devoted in recent decades to the search for practical technologies that can overcome some or all of the limitations mentioned.
One suggested approach is to esterify and acetalize biooil with alcohols like ethanol and butanol (e.g. EP0718392 and Upgrading of Flash Pyrolysis Oil by Reactive Distillation Using a High Boiling Alcohol and Acid Catalysts, F. H. Mahfud, I. Melián-Cabrera, R. Manurung and H. J. Heeres, Trans. IChemE, Part B, 85 (B5) 466-472, 2007). However the reaction products still have high acidity and significant water content, while the increase in specific heating value is modest. Furthermore the products themselves also tend to be chemically unstable and reactive.
Another approach is to emulsify the biooil in diesel fuels using suitable surfactants (e.g. U.S. Pat. No. 5,820,640 and Development of emulsions from biomass pyrolysis liquid and diesel and their use in engine—Part 1: emulsion production, D. Chiaramonti et al, Biomass and Bioenergy 25, 85-99 (2003); Development of emulsions from biomass pyrolysis liquid and diesel and their use in engines—Part 2: tests in diesel engines, D. Chiaramonti et al, Biomass and Bioenergy 25, 101-11, (2003)). While this resolves the problem of compatibility with industrial hydrocarbon fuels, it introduces new issues such as emulsion stability while problems related to chemical stability and corrosivity remain.
In yet another approach nascent uncondensed biooil is deoxygenated by conducting it over zeolite catalysts to directly produce low molecular weight aromatics like BTX (benzene, toluene, xylene) from biooil (e.g. U.S. Pat. No. 4,308,411, 1981 and Green Gasoline by Catalytic Fast Pyrolysis of Solid Biomass Derived Compounds, T. R. Carlson, T. P. Vispute, and G. W. Huber, ChemSusChem, 1, 397-400 (2008)). However zeolite catalysts are acidic and oxygen is removed largely by dehydration to give water. Consequently, on account of the inherent molar deficiency of hydrogen relative to oxygen and carbon in biooil, yields are relatively low and coke formation is high, adversely affecting the technological difficulty and economic performance.
An indirect approach involves gasification of the biooil (and/or the char co-product) to syngas and subsequent Fischer-Tropsch synthesis of long chain hydrocarbons or olefins from the syngas in a so-called Biomass To Liquids (BTL) process (e.g. the Bioliq process described by Henrich et al. (Cost estimate for biosynfuel production via biosyncrude gasification, E. Henrich, N. Dahmen and E. Dinjus, Biofuels, Bioprod. Bioref. 3:28-41 (2009)). However overall yields of hydrocarbons from biomass are rather low and capital costs are high. Furthermore the minimum scales at which BTL processes are predicted to be economic are large relative to the typical local availability of biomass, which necessitates complex logistics for biomass delivery with substantial transportation costs.
Over the last two decades, the approach of direct hydroprocessing of biooil to convert it to stable oxygenates or hydrocarbons has been studied intensively. Elliott has published a comprehensive review of these many historical efforts, including work with model compounds known to be present in biooil (Historical Developments in Hydroprocessing BioOils, D. C. Elliott, Energy & Fuels 2007, 21, 1792-1815).
A major obstacle to the catalytic hydroprocessing of biooil has been its propensity to polymerize under heating above about 100° C., leading ultimately to the formation of extraneous solids or coke at temperatures above about 140° C., with consequences like reactor plugging and catalyst deactivation.
Pyrolytic Lignin
These difficulties can be partially circumvented by hydroprocessing only the thermally resistant portion of the biooil. Thus by adding water to biooil it can be separated into an aqueous phase and, usually between 20 and 30% of, a viscous higher density phase, so-called pyrolytic lignin as it is largely derived from the lignin fraction of the biomass pyrolysis feedstock. Since pyrolytic lignin is rich in phenolic material it has much greater thermal stability than the carbohydrate derived portion of the biooil and consequently is easier to catalytically hydroprocess without solids formation. This was the approach taken by Piskorz et al (Conversion of Lignins to Hydrocarbon Fuels, J. Piskorz, P. Majerski, D. Radlein, and D. S. Scott, Energy & Fuels, Vol 3, 723-726, 1989) and more recently by Marker and Petri, (U.S. Pat. No. 7,578,927, 2009). However in this case one must confront the problem of what to do with the greater, water soluble, portion of the biooil.
Whole Biooil
In order to hydroprocess whole biooil, Elliott et al (U.S. Pat. No. 4,795,841, 1989) proposed to minimize these problems by a two-staged process in the first stage of which the overall thermal stability of the biooil is enhanced by catalytic hydrogenation at a low temperature (˜280° C.).
More recent progress in biooil hydroprocessing is illustrated by the work of Heeres et al (Hydrotreatment of Fast Pyrolysis Oil Using Heterogeneous Noble-Metal Catalysts, J. Wildschut, F. H. Mahfud, R. H. Venderbosch, and H. J. Heeres, Ind. Eng. Chem. Res., 48 (23), 10324-10334 (2009)), who reported batch experiments in which a comparison was made between mild hydrogenation and deep hydrotreating of biooil over various noble metal as well as conventional Co—Mo and Ni—Mo hydrotreating catalysts. Mild hydrogenation at 250° C. and 100 bar gave single phase liquid products with oxygen contents variously between 18 to 27% and in yields between 21 and 55 mass % (dry basis). However substantial solid by-products (char/coke) were formed while the oxygen content of these oils remained high, between 18 and 27 mass %. On the other hand deep hydrotreatment at 350° C. and 200 bar using gave varying degrees of solid by-products along with one or more oil phases with an average oxygen content of 5 to 11 mass %. They concluded that on the basis of oil yields, deoxygenation levels, and extents of hydrogen consumption, Ru/C seemed to be the most promising catalyst for further testing.
Under deep hydrotreatment the hydrogen consumption for this catalyst was estimated to be about 3.6 mass % of the biooil on a dry basis. The lighter of the two products oils had a density of 0.9 g/cm3, water content of 1.5 mass %, oxygen content of 4.8 mass % and a higher heating value of 42.6 MJ/kg.
These results indicate that besides the issues of solids formation and rapid catalyst deactivation, biooil hydroprocessing is also rendered difficult by the formation of multiple oil phases besides the aqueous phase. In addition, the kinetics of hydrogenation are slow, especially at the relatively low temperatures required for thermal stabilization of biooil, and moreover, hydrogen consumption is high.
Baldauf et al. (W. Baldauf, U. Balfanz and M. Rupp, Upgrading of flash pyrolysis oil and utilization in refineries, Biomass and Bioenergy, Vol. 7, pp. 237-244, 1994) described the direct hydrodeoxygenation of a flash pyrolysis biooil over commercial CoMo and NiMo catalysts in a packed bed reactor and reported that “the process is restricted by several operational problems such as rapid catalyst deactivation, coking and plugging”. Biooil has also been co-processed with a hydrocarbon solvent. Thus, as reported by Elliott (Historical Developments in Hydroprocessing BioOils, D. C. Elliott, Energy & Fuels 2007, 21, 1792-1815) Churin et al. (Churin, E.; Grange, P.; Delmon, B. Quality Improvement of Pyrolysis Oils; final report on contract no. EN3B-0097-B for the Directorate-General Science, Research and Development, Commission of the European Communities, 1989) co-processed biooil in a 1:1 ratio with the hydrogen donor solvent tetralin. They concluded that this led to a marked improvement in the quality of the product, and the catalysts were less deactivated by coke deposition, which was attributed to the hydrogen donor properties of the solvent.
US2009/0253948 discloses a process for the conversion of biomass derived pyrolysis oil to liquid fuel by two-stage deoxygenation of the pyrolysis oil and separation of the products and in which the final hydrocarbon product may be recycled. However it does not teach the high dispersal or solubilization of pyrolysis oil in a hydrocarbon medium with the consequent benefits of a large increase in reaction rates and catalyst lifetime.”
Nevertheless an efficient hydrotreating process has yet to emerge. The difficulties that have been encountered have their root in the rapid thermal polymerization of biooil that leads to fast catalyst deactivation. In other words, at the temperatures typically required for hydroprocessing of biooil, polymerization reactions are substantially faster than the competing hydroprocessing reactions, ultimately leading to coke formation.
Indeed, in a recent study of biooil stabilization by hydrogenation, Venderbosch et al (Stabilization of biomass-derived pyrolysis oils, R. H. Venderbosch, A. R. Ardiyanti, J. Wildschut, A. Oasmaa and H. J. Heeres, J. Chem. Technol. Biotechnol. 2010, 85: 674-686) 2010) came to similar conclusions, stating that: “In hydroprocessing of biooils, a pathway is followed by which pyrolysis oils are further polymerized if H2 and/or catalyst is absent, eventually to char components, or, with H2/catalyst, to stabilized components that can be further upgraded.”
Therefore, what is required is an improved process for biooil hydroprocessing that minimizes the formation of solids and catalyst deactivation, minimizes hydrogen consumption and maximizes space-time yield of deoxygenated oil product. It is also desirable to maximize the fraction of hydrocarbon product that boils in the range of useful fuels like gasoline or diesel. A preferred process would have the further following desirable characteristics:                1) Convert biooil to high value hydrocarbon products, especially motor fuels.        2) Minimize hydrogen consumption.        3) Operate at conditions that minimize the coke formation that leads to catalyst deactivation and/or reactor plugging.        4) Maximize reaction kinetics to reduce required reactor volumes and associated capital and operating costs and maximize throughput/productivity.        5) Make operating conditions as mild as possible to further reduce capital and operating costs.        6) The process should accommodate a wide range of biomass-derived liquids, and biomass pyrolysis liquids in particular, that may have wide ranges of viscosity, water content and degree of polymerization depending on the feedstock and pyrolysis process.        7) Any catalyst employed should be low-cost and long-lived.        8) Preferably it should be possible to implement the process with existing reactor technologies.        9) Preferably the method should accommodate co-processing with petroleum derived hydrocarbon feedstocks for compatibility with oil refineries.        
The inventive process disclosed herein meets all these criteria.